Optical Spectroscopy (IR)

Question 1

Instruments

Answer 1

Light source, wavelength selection, sample and light detector

Question 2

IR Sources

Answer 2

Globar or a Nernst Glower

Question 3

Detectors (Types)

Answer 3

Thermal, Pyroelectric and Photoconducting

Question 4

Detectors (Thermal)

Answer 4

Thermocouples (bi-metallic). Different wires joined at one end and generate thermoelectric voltage between open pair according to difference in temperature between ends

Question 5

Detectors (Pyroelectric)

Answer 5

Pyroelectric material is sandwiched between two electrodes. IR radiation heats up pyroelectric material and therefore capacitance of detector increases

Question 6

Detectors (Photoconducting - MCT)

Answer 6

Photovoltaic detectors (MCT - Mercury Candium Telluride) where IR radiation promotes non-conducting electrons to a higher energy state which reduces bulk resistance of device

Question 7

FT-IR Scheme

Answer 7

Source -> Interferometer -> Sample -> Detector -> Recorder.

FT-IR Machines were very large and very expensive

Question 8

Interferometer

Answer 8

Michelson Interferometer commonly used. Allows simultaneous determinations of intensity of light at all wavelengths. Fourier transform deconvolutes the interference pattern caused by interferometer (beam A and B)

Question 9

Benefits of FT-IR

Answer 9

Multiplex advantage, less moving parts (no monochromator), very fast, high resolution, high sensitivity and excellent precision/accuracy